This is an old project I started just after releasing the mounted guns a couple of months ago. I got the idea of a buggy with a machinegun from playing BF2, and went into mounting it on the HL2 buggy frame, along with some extra stuff:

The buggy. It has a .50 machinegun mounted to its top, you can stand on a platform on the back and fire it.
Even NPC's can use it! NPC's might not seem to grab the gun properly, but how could I possibly fix that?

Here is a neat detail I decided to add. Every rebel veichle should have its own box of supplies! "+use"-ing the small radio unit here will bring out the crate with health/ammo onto the platform.
Shoot (crowbar or any other weapon) at an item inside the box to "take it" (spawn it). Use the radio unit again to push the box back again.
You can spawn an unlimited amount of supplies, however I reccomend you to only use supplies when you can actually pick them up.
The little button spawns a horde of zombies.
A rebel is nearby. He will seek to step up onto the platform and take control of the gun. He is very skilled with it! He cannot stand there while you drive it around, unfortunately. Maybe if you place this in a multiplayer map, perhaps your friend could act as a gunner?

Hehe ok, keep your pants on I was thinking about a co-op map or something in a desert/similar where players can fight antlions or zombies/whatev, using buggies like this one as mobile bases.
I'm going to be away for the summer, so I can't start any new map in the next 5 weeks or so :S